1. Field of the Invention
The present invention relates to a surface acoustic wave filter, and a composite surface acoustic wave filter composed of a surface acoustic wave filter and a one-port surface acoustic wave resonator, and more particularly to a surface acoustic wave filter having a low insertion loss in the RF band, which is suitable for use in a mobile communication system or the like, a composite surface acoustic wave filter, and a mobile communication system which employs these filters.
2. Description of the Related Art
(Interdigitated interdigital surface acoustic wave filter with normal transducers)
FIG. 19 of the accompanying drawings shows a conventional interdigitated interdigital surface acoustic wave filter 11 with normal transducers on a substrate 111. If the number of transducers is indicated by (2m+1), then the bidirectional loss BL (dB) of the interdigitated interdigital surface acoustic wave filter 11 is represented by: EQU BL=10 log{(2m+2)/2m}(dB) (1)
The bidirectional losses BL for the different numbers of transducers are given in the following table:
______________________________________ 2m + 1 BL (dB) ______________________________________ 5 1.80 7 1.25 9 0.97 11 0.70 13 0.67 ______________________________________
Since the interdigitated interdigital surface acoustic wave filter 11 shown in FIG. 19 has 5 transducers, its bidirectional loss BL is 1.80 dB. It can be seen from the table that increasing the number of transducers is effective to reduce the bidirectional loss BL.
The input admittance Y of the transducers is expressed by the following equation (2): EQU Y=Ga+jBa+j.omega.C.sub.T ( 2)
where EQU Ga=Ga'(sin x/x).sup.2 ( 3) EQU Ba=Ga'(sin 2x-2x)/2x.sup.2 ( 4) EQU Ga'=4k.omega..sup.2.sub.O C.sub.S N.sup.2 /.pi. (5) EQU x=N.pi.(.omega.-.omega..sub.O)/.omega..sub.O ( 6) EQU C.sub.T =NC.sub.S ( 7) EQU C.sub.S =.epsilon..sub.O .epsilon..sub.r W (8) EQU .omega.=2.pi.f EQU .omega..sub.O =2.pi.f.sub.O
and C.sub.S represents the capacitance per transducer port, f the frequency, f.sub.O the center frequency, N the number of electrode finger pairs, W the aperture length, .epsilon..sub.O the dielectric constant of vacuum, .epsilon..sub.r the dielectric constant of the substrate, and k the electromechanical coupling coefficient.
The above equations indicate that as the number of electrode finger pairs increases, the radiation conductance Ga increases and the input impedance decreases. Since the transducers are electrically connected parallel to each other in the interdigital configuration, the input impedance is lower as the number of transducers (2m+1) is greater.
To reduce the passband of a surface acoustic wave filter, the number of electrode finger pairs of each transducer is increased. Therefore, the input impedance of each transducer is reduced, making it impossible to increase the number of interdigital transducers due to the impedance limitation. As a consequence, the bidirectional loss of the surface acoustic wave filter is increased, resulting in a greater insertion loss.
For example, it is assumed that the number of electrode finger pairs per input transducer is 22, the number of electrode finger pairs per output transducer is 30, and the aperture length is 20.lambda. (.lambda. is the wavelength of the input signal) in the interdigitated interdigital surface acoustic wave filter 11 with the 5 transducers (the electrode finger pairs are shown as fewer than actual in FIG. 19). The impedances of such interdigitated interdigital surface acoustic wave filter 11 are calculated, and shown in FIGS. 20A and 20B with respect to normalized frequencies ranging from 0.9 to 1.1. FIG. 20A shows the calculated impedances on the input transducers, and FIG. 20B shows the calculated impedances on the output transducers. For a 50 .OMEGA.-impedance arrangement, the input and output sides can be matched by using a matching circuit shown in FIG. 21A.
As described above, the bidirectional loss of an interdigitated interdigital surface acoustic wave filter with 5 transducers is 1.80 dB (see the above table). Lowering the bidirectional loss requires that the number of transducers be increased. If the number of transducers is increased, however, the impedances are reduced. For example, a surface acoustic wave filter 12 with 13 transducers on a substrate 121 as shown in FIG. 22 has impedances as shown in FIGS. 23A and 23B with respect to normalized frequencies ranging from 0.9 to 1.1. FIG. 23A shows the calculated impedances on the input transducers, and FIG. 23B shows the calculated impedances on the output transducers. The impedances which are lowered can be matched in a passband by using a 4-element matching circuit shown in FIG. 21B which includes capacitors 32I, 32O added to the matching circuit shown in FIG. 21A. As a result, the surface acoustic wave filter 12 has insertion loss vs. frequency characteristics as shown in FIG. 24. While the bidirectional loss of the surface acoustic wave filter 12 is about 1.0 dB lower than that of the surface acoustic wave filter with 5 transducers, the number of matching elements required is increased.
As shown in FIG. 24, the insertion loss vs. frequency characteristics of the surface acoustic wave filter 12 with normal transducers to which the matching circuit shown in FIG. 21B is connected suffer large side lobes outside of the passband. For suppressing such large side lobes, it is necessary to weight the transducers. However, although the side lobes of a surface acoustic wave filter with weighted transducers is suppressed, the surface acoustic wave filter has a widened trap frequency band as indicated by the arrows in FIG. 24.
FIG. 25 shows an interdigitated interdigital surface acoustic wave filter 10 which employs different withdrawal-weighted transducers for suppressing out-band side lobes. As shown in FIG. 25, the interdigitated interdigital surface acoustic wave filter 10 has 13 transducers on a substrate 131. FIG. 26 shows insertion loss vs. frequency characteristics of the surface acoustic wave filter 10 with the matching circuit shown in FIG. 21B being connected thereto. While the side lobes in the insertion loss vs. frequency characteristics shown in FIG. 26 are smaller than those in insertion loss vs. frequency characteristics shown in FIG. 24, the attenuation in the vicinity of the passband is lowered due to a widened trap frequency band.
As shown in FIG. 27, there has also been known an transducer 14 with an increased number of electrode finger pairs on a substrate 141 for use with surface acoustic waves. The impedance of the transducer 14 with many electrode finger pairs exhibits resonant characteristics as shown in FIG. 28. It is known that when the transducer 14 or resonator is inserted in series with a circuit, it provides a stop band at an antiresonant frequency. Since the transducer 14 functions as a capacitive element in the passband, the impedance is low and the loss is small if the capacitance of the capacitive element is large. However, since there is usually a limitation on the capacitance, the impedance is prevented from being reduced as desired, causing an undue loss.
(Two-port surface acoustic wave resonator filter)
As shown in FIG. 29, a conventional two-port surface acoustic wave resonator filter 20 comprises an input transducer 21, two output transducers 22a, 22b disposed one on each side of the input transducer 21 and electrically connected parallel to each other, and two reflectors 23a, 23b disposed outside of the output transducers 22a, 22b, respectively. These transducers are formed on one substrate. FIG. 30 illustrates calculated insertion loss vs. frequency characteristics of the two-port surface acoustic wave resonator filter 20. The insertion loss vs. frequency characteristics shown in FIG. 30 were calculated when the substrate was made of 64y-xLiNbO.sub.3, the number of input electrode finger pairs was 18.5, the number of output electrode finger pairs was 12.5, and the aperture length was about 60.lambda. where .lambda. is the wavelength of the input signal.
The two-port surface acoustic wave resonator filter 20 suffers a low insertion loss, and has good attenuation characteristics in a frequency band remote from the passband. However, the two-port surface acoustic wave resonator filter 20 essentially gives rise to a side lobe in a frequency range near and higher than the passband.
To avoid the above difficulty, another conventional two-port surface acoustic wave resonator filter 40 shown in FIG. 31 is composed of the two-port surface acoustic wave resonator filter 20 having the input transducer 21, the output transducers 22a, 22b, and the reflectors 23a, 23b, and another two-port surface acoustic wave resonator filter 30 having an input transducer 31, the output transducers 32a, 32b, and reflectors 33a, 33b that are arranged similarly to the two-port surface acoustic wave resonator filter 20, the two-port surface acoustic wave resonator filters 20, 30 being mounted on one substrate and connected in cascade. The two-port surface acoustic wave resonator filter 40 achieves a large out-band attenuation level.
As shown in FIG. 32, still another conventional two-port surface acoustic wave resonator filter 50 comprises an input transducer 51, an output transducer 52, a reflector 53a disposed outside of the input transducer 51, and a reflector 53b disposed outside of the output transducer 52. These transducers are formed on one substrate. The two-port surface acoustic wave resonator filter 50 has calculated insertion loss vs. frequency characteristics as shown in FIG. 33. As with the two-port surface acoustic wave resonator filter 20, the two-port surface acoustic wave resonator filter 50 has poor attenuation characteristics in a frequency range near and higher than the passband. The insertion loss vs. frequency characteristics shown in FIG. 33 were calculated when the substrate was made of x-112yLiTaO.sub.3, the number of input electrode finger pairs was 50, the number of output electrode finger pairs was 50, and the number of reflectors on each side was 100.
The two-port surface acoustic wave resonator filter 40 has an out-band attenuation level which is twice the out-band attenuation level of the two-port surface acoustic wave resonator filter 20, but suffers a doubled insertion loss. If the attenuation level is not sufficient, three or four two-port surface acoustic wave resonator filters are connected in cascade. Therefore, since a two-port surface acoustic wave resonator filter itself is unable to suppress a limited side lobe in a frequency range near and higher than the passband, a plurality of two-port surface acoustic wave resonator filters have to be connected in cascade to suppress such a side lobe. However, the cascaded two-port surface acoustic wave resonator filters undergo an increased insertion loss.
Consequently, although a two-port surface acoustic wave resonator filter has good attenuation characteristics in other frequency ranges than a frequency range near and higher than the passband, other two-port surface acoustic wave resonator filters have to be connected in cascade to the two-port surface acoustic wave resonator filter only to maintain a desired level of attenuation in the frequency range near and higher than the passband.
(Interdigitated interdigital surface acoustic wave filter with a plurality of different withdrawal-weighted transducers)
An interdigitated interdigital surface acoustic wave filter which employs a plurality of different withdrawal-weighted transducers is shown in FIG. 25. As shown in FIG. 25, the interdigitated interdigital surface acoustic wave filter 10 comprises a plurality of different withdrawal-weighted input transducers 11a, 11b, 11c, 11d, 11c, 11b, 11a disposed on a substrate 13 of 36.degree.y-xLiTaO.sub.3, and a plurality of different withdrawal-weighted output transducers 12a, 12b, 12c, 12c, 12b, 12a disposed on the substrate 13 between the input transducers. The input transducers 11a, 11b, 11c, 11d, 11c, 11b, 11a are electrically connected parallel to each other and also connected to an input terminal A, and the output transducers 12a, 12b, 12c, 12c, 12b, 12a are electrically connected parallel to each other and also connected to an output terminal B.
The calculated insertion loss vs. frequency characteristics of the interdigitated interdigital surface acoustic wave filter 10 are shown in FIG. 26. When the insertion loss vs. frequency characteristics were calculated, an inductive element is connected as a matching circuit parallel to the interdigitated interdigital surface acoustic wave filter 10. In the illustrated insertion loss vs. frequency characteristics, the level of attenuation in a stop band near and lower than the passband is about 15 dB, and the level of attenuation in a stop band near and higher than the passband is about 25 dB. While the interdigitated interdigital surface acoustic wave filter 10 has a low insertion loss in the passband, the levels of attenuation outside of the passband are not enough. It is also difficult to achieve sharp cutoff characteristics in the insertion loss vs. frequency characteristics.
Improved out-band insertion loss vs. frequency characteristics can be accomplished by interdigitated interdigital surface acoustic wave filters with withdrawal-weighted transducers. However, the trap frequency band as shown in FIG. 26 is widened though the side lobes in the insertion loss vs. frequency characteristics are suppressed.
A filter composed of surface acoustic wave resonators 54.about.58 connected in a ladder configuration as shown in FIG. 34 has insertion loss vs. frequency characteristics as shown in FIG. 35. Such a ladder-type filter arrangement requires that the product of the admittance of the surface acoustic wave resonators 55, 57 connected parallel to each other and the impedance of the surface acoustic wave resonators 54, 56, 58 connected in series to each other be 1 or more outside of the passband. Thus, the surface acoustic wave resonators 54, 56, 58 connected in series to each other and the surface acoustic wave resonators 55, 57 connected parallel to each other have to be related to each other in a certain manner.
As described above, if the transducers of the conventional interdigitated interdigital surface acoustic wave filters are withdrawal-weighted in order to suppress the side lobes, then the trap frequency band is increased, impairing the cutoff characteristics in the insertion loss vs. frequency characteristics.
It has been customary to connect interdigitated interdigital surface acoustic wave filters in cascade for increasing the level of attenuation in stop bands. This approach is effective to increase the out-band attenuation levels, but also suffers an increased insertion loss in the passband.
Filters composed of surface acoustic wave resonators connected in a ladder configuration require that a certain relationship be achieved between those surface acoustic wave resonators which are connected in series to each other and those surface acoustic wave resonators which are connected parallel to each other. As a result, the levels of attenuation in the stop bands remote from the passband are low, and there are certain limitations imposed on the passband and notch frequency. Higher attenuation levels outside of the passband sacrifice the insertion loss, i.e., cause an increase in the insertion loss.